A rotor of the induction motor is proposed (for example, refer to patent document 1), to reduce eddy current loss of the rotor in which a squirrel-cage conductor is formed by die-casting aluminum into rotor slots. Two types of rotor slots including the inner peripheral side rotor slots and the outer peripheral side rotor slots are arranged in the radial direction of a rotor core, aluminum or an aluminum alloy is filled into only the inner peripheral side rotor slots, and a rotor bar which forms a squirrel-cage conductor is formed. The outer peripheral side rotor slots are formed as empty spaces without fillings, so that a substance having low electric resistance does not exist near the surface of a rotor. As a result, an eddy current is not generated, and a loss due to a PWM (pulse width modulation) can be reduced sharply. Thereby, the efficiency, especially the efficiency in a low load, of a PWM driven induction motor is enhanced.
Another rotor of the double squirrel-cage induction motor is proposed (for example, patent document 2), having a slot portion which includes two types of outer slots (large and small), both are the same in numbers, which are arranged alternately and concentrically along an outer peripheral edge of the rotor at equal intervals in parallel with the rotation axis, and inner slots which extend internally in the radial direction, being continuous to an inner end of the small slots of the outside slots radially. The inner slots are projected circumferentially at the intermediate section, and the cross-sectional area increases remarkably.    Patent document 1: Japanese Laid-Open Patent Publication No. HEI 08-140319    Patent document 2: Japanese Laid-Open Patent Publication No. HEI 01-129738